1. Field of the Invention
The present invention relates generally to a chemical-mechanical polishing (CMP) process and more specifically to a process wherein the rate of polishing at different locations on the wafer is monitored and the force which is applied between the pad and wafer is varied in a timed relationship with the effects of the CMP in a manner to reduce the polishing differential and to increase the flatness of the layer(s) on the upper surface of wafer.
2. Description of the Related Art
CMP is a method of planarizing or polishing semiconductor and other types of substrates. At certain stages in the fabrication of devices on a substrate, it becomes necessary to polish the surface of the substrate before further processing may be performed. One polishing process, which passes a conformable polishing pad over the surface of the substrate to perform the polishing, is commonly referred to as mechanical polishing. Mechanical polishing may also be performed with a chemically active abrasive slurry, which typically provides a higher material removal rate, and a higher chemical selectivity between films of the semiconductor substrate, than is possible with mechanical polishing. When a chemical slurry is used in combination with mechanical polishing, the process is commonly referred to as CMP.
One prior art CMP process is disclosed in U.S. Pat. No. 5,234,867, Schultz. That process generally includes the steps of rotating a polishing pad which has a diameter several times larger than a substrate, pouring a chemical slurry on the rotating polishing pad, and placing a substrate on the rotating polishing pad and independently rotating the substrate while maintaining pressure between the rotating polishing pad and the substrate. The polishing pad is held on a relatively massive planer platen which is coupled to a motor. The motor rotates the platen and polishing pad, and the platen provides a flat surface to support the rotating polishing pad. To independently rotate the substrate, it may be located within a separate rotating polishing head or carrier, which is also moveable in an x-y plane to locate the substrate rotating therein in specific positions on the large, rotating platen. As the polishing pad is several times larger than the substrate, the substrate may be moved from the outer diameter to the center of the rotating polishing pad during processing.
The rate of material removed from the substrate in CMP is dependent on several factors, including among others, the chemicals and abrasives used in the slurry, the surface pressure at the polishing pad/substrate interface, the net motion between the substrate and polishing pad at each point on the substrate. Generally, the higher the surface pressure, and net motion at the regions of the substrate which contact the polishing pad, the greater the rate of material removed from the substrate. In Schultz, '867, the removal rate across the substrate is controlled by providing an irregularly-shaped polishing pad, and rotating the substrate and polishing pad to attempt to create an equal "residence time" of the polishing pad against all areas of the substrate, and in one embodiment thereof, by also varying the pressure at the substrate/polishing pad interface.
Using a large rotating polishing pad for CMP processing has several additional processing limitations which lead to non-uniformities in the polished substrate. Because the entire substrate is rotated against the polishing pad, the entire surface of the substrate is polished to a high degree of flatness as measured across the diameter of the substrate. Where the substrate is warped, the portions of the substrate which project upwardly due to warpage tend to have higher material removal rates than the remainder of the substrate surface. Further, as the polishing pad polishes the substrate, material removed from the substrate forms particulate which may become trapped in the pad, and the polishing slurry dries on the pad. When the pad becomes filled with particulates and the slurry dries in the pad, the polishing surface of the pad glazes and its polishing characteristics change. Unless the user constantly monitors the removal rate of the polishing pad with each substrate, or group of substrates, and adjusts the slurry, load, position, and/or rotation speed of the polishing pad or substrate to maintain the desired material removal rate, the amount of material removed by the polishing pad from each substrate consecutively processed thereon will decrease.
In an effort to overcome the above drawbacks, it has been proposed in U.S. Pat. No. 5,944,582 issued on Aug. 31, 1999 in the name of Talieh, to polish substrates using an arrangement wherein the polishing pad is no larger than, and is preferably substantially smaller than, the radius of the substrate being polished. In one arrangement disclosed in this reference, the apparatus includes a rotating plate on which a substrate is held, and a polishing arm, which is located adjacent the plate, is moved across the surface of the substrate as the substrate rotates on the rotating plate. The polishing arm includes a polishing pad on the end thereof, which is preferably variably loadable against the surface of the substrate as different areas of the substrate are polished thereby. The speed of rotation of the substrate may be varied, in conjunction with, or independently of, any adjustment in the load of the polishing pad against the substrate to control the rate of material removed by the polishing pad as it crosses the substrate.
However, even with this arrangement a problem occurs in that the chemical effects and the mechanical effects of the polishing are not the same and vary in a manner which causes a relatively large variation in polishing rate from place to place on a wafer surface. This variation can, for example in the case of controlling the thickness of a dielectric layer affects interconnection capacitance and thus circuit performance. However, at this time, it is extremely difficult to achieve planar uniformity across a wafer.